The quality of fuels with lower emissions is very important owing to more and more restrictive fuel legislations in the world. High research octane number (RON) will need to be maintained, while the aromatic content will be reduced below 35 vol % level. Aromatic compounds have the greatest contributions to RON of gasoline. Reducing the content of aromatics will cause significant reductions in the quantity of gasoline and RON deficiencies. In addition, as aromatic compounds have a lower vapor pressure, the volatility of the gasoline will increase with the reduction of aromatics. Consequently, the cost of gasoline production will increase to maintain the acceptable level of Reid Vapor Pressure (RVP) since the blending of light hydrocarbons such as C4-C5, which are relatively cheap, will have to be reduced. Methyl-tertiary-butyl-ether (MTBE) provides a considerable octane supply to the gasoline and contributes a significant diluting effect owing to its high blending concentrations (10-15%). For the past twenty years or so, gasoline sold in the United States and many other countries has been blended with up to 15% volumes of MTBE, an oxygenate, in order to raise the octane rating and to reduce environmentally harmful exhaust emissions. Unfortunately, MTBE is itself a pollutant, having an objectionable and strong odor and taste at extremely low concentrations (ppb) and having been classified as a potential human carcinogen. The concentration of undesired components, such as, benzene, aromatics, sulfur will be reduced when 15% volumes of MTBE is blended. If MTBE is removed without adding other diluents, this beneficial dilute effect would be lost.
One replacement for MTBE is fermented grain ethanol from wheat or sugar cane. U.S. Pat. No. 4,398,920(A) describes a blended fuel comprising a mixture of ethanol, acetone, and methanol from fermentation. Knowing the toxicology of ethanol and that ethanol provides a higher blending RON shows advantages of ethanol. However, the production of ethanol is costly and it is economically sustainable only when tax reductions are granted. Furthermore, producing sufficient quantities of grain ethanol to satisfy the needs of the transportation industry will compete with limited food supplies. Further, ethanol has relatively low energy content when compared to gasoline. Ethanol contains about 76,000 Btu's/gallon while gasoline contains about 113,000 Btu's/gal. Also, ethanol has high affinity towards water and it cannot mix together with the gasoline directly in the refinery but is only added just before the last distribution point in the network. Moreover ethanol easily forms low-boiling azeotropic mixtures with the components of gasoline which leads to higher RVP which varies from 17 to 22 psi at 10-15% blending levels. However, high vapor pressure is a problem especially in the summer months. In addition, excessively high concentrations of ethanol (about 10% of ethanol by volume) seem to cause increase in the emissions of NOR.
The use of ethanol could provide various advantages such as known toxicology and higher blending RON. However, ethanol usages are limited by the costly production, relatively low energy content, high affinity towards water, on-site blending limitations and higher RVP.
Therefore for reasons brought forth above, an effective replacement for MTBE and ethanol in gasoline as an octane enhancer is needed to raise the octane ratings of gasoline and reduce deficits from their use.
Mixed butenes are often used as feeds for an alkylation process. The production of alkylates are not however an environmentally friendly process because of the mixed spent acids. In addition, alkylates have lower RON and Octane Sensitivity. Therefore an effective replacement for alkylates, MTBE and ethanol in gasoline is needed to raise the octane ratings of gasoline. The use of high-octane hydrocarbon components could enable the problems described herein above to be overcome. Further, C8 olefins as RON enhancers show many advantages over currently used octane enhancers, such as MTBE or ethanol, and alkylates.
Alkylates which include iso-octane and trimethyl pentanes are extremely desirable for their higher RON, low RVP and their positive influence on emissions. Alkylation is a refinery process which consists of the formation of highly branched paraffins by the catalytic alkylation reaction of isobutane with light olefins such as propylene and butenes in the presence of H2SO4 or HF. From an environmental point of view, both H2SO4 and HF are strong acids. The handling of enormous volumes of H2SO4 or HF in routine operations, disposals of its by-products and transporting the acid for its recovery are high risk owing to their corrosive nature. The production of alkylates is not an environmental friendly process. In addition, alkylates have Motor Octane Number (MON) comparable to their RON and hence have lower octane sensitivity. In modern and future high efficiency, spark ignition engines, fuels with higher octane sensitivities for a given RON will have better anti-knock quality and allow higher combustion efficiency.
Therefore, there is strong incentive for the use of high-octane hydrocarbon components deriving from the dimerization or oligomerization of butenes which can overcome the problems described above and show many advantages over currently used, MTBE, ethanol or alkylates as RON enhancers. The major compounds obtained from the dimerization/oligomerization of mixed butenes are C8 and C12, olefins. Among butene dimers, diisobutenes (DIBs) is the most preferred one which is a non-oxygenative fuel component with many advantages such as higher RON, higher octane sensitivity or better anti-knock quality, higher energy content compared to MTBE and alkylates, lower RVP than MTBE and ethanol. Highly branched octenes have a number of advantages including: giving very similar RON increases as MTBE when the same volume is added to a low RON gasoline; having higher RON sensitivities compared to MTBE and alkylates and hence, will have a better anti-knock quality and higher combustion efficiency in modern and future spark ignition engines and having a higher energy content compared to MTBE and lower RVP, while not increasing RVP like MTBE and ethanol. Further, at 15% blending volume to a 91 RON gasoline, DIBs will have about 2.8% more energy per liter over MTBE. Thus on average, there will be a saving of 2.8% by volume of total gasoline consumption.
There are mainly two types of light aliphatic olefin dimerization mechanisms, one is acid-catalyzed ionic mechanism and another is metal-catalyzed coordinative mechanism. The acid catalyzed processes provide strongly branched olefins while the coordinative metal complex-catalyzed processes produce largely linear olefins or mainly head-tail and head-head dimerization and oligomerizations of double bonds of the olefins. Strongly branched octenes such as dimethyl-hexene and trimethyl-pentene have high Research Octane Number (RON), so acidic catalysts are mostly welcomed for producing gasoline enhancers. On the other hand, the coordinative metal complex-catalyzed processes often use Ziegler type catalysts, nickel, cobalt, iron complexes based catalysts, titanium, and zirconium based single site catalysts. Aluminum base Lewis acid catalyst and supported nickel oxides give predominantly linear type of olefins. The coordinative metal complex-catalyzed processes have been studied extensively. Such processes, both homogeneous and heterogeneous, are normally complicated and require moisture and air free environment. The coordinative metal complex-catalyzed processes normally use 1-butene as starting material as well.
Heteropolyacids (HPAs) such as H3PW12O40 and H3PMo12O40 are strong Brønsted acids. According to a Hammett acidity function, the acidity of H3PW12O40 is less than −8.2 and that will put such acids into a super-acid region.
Therefore, HPA is used as a catalyst in various reactions due to its high acidity and oxidative nature. The first industrial process using a HPA based catalyst was for the hydration of propylene to isopropanol in aqueous solutions.
Olefin dimerizations or oligomerizations are industrial important reactions to convert light olefins (C2-C5) into higher olefins often catalyzed by strong acids. The products can be used as feedstocks for other valuable petrochemicals. The Bayer process operates at 100° C. with an ionic exchange resin catalyst to give 75% of dimers and 25% of trimers with high conversion ratio from isobutene. The dimerization and oligomerization of linear butene with UOP's Octol process using phosphoric acid physically absorpted on a support or IFP's Dimersol X process based on a nickel salt/aluminum alkyls catalyst gave less branched octenes which are useful for the preparations of plasticizers. Selective dimerization of isobutene are particular interested by industry since the separation of individual butene isomers appear difficult especially for isobutene and 1-butene which almost have identical boiling points. One method to separate isobutene from mixed butenes involves the selective dimerization or oligomerization of isobutene. HPA dissolved in water has been reported as effective catalysts for such process. Traces amount of isobutene in C4 mixtures can be removed by such HPA or its salts such as Cr3+, La3+, Al3+, Fe3+, Cu2+, Cd2+, Ca2+, and Zn2+ through selective isobutene dimerizations as well. However, due to the corrosive nature of HPA, the processes using the unsupported HPA often require high maintenances. Solid acid or supported catalysts are preferred for industrial processes since such process is less hazardous and more easily to be operated. Unfortunately, when HPA or its water soluble salts is supported directly, on SiO2 for example, the acid strength decreases. Therefore, a lower activity using supported HPA catalysts for olefin dimerization is expected. Such supported HPA acids have been reported in the literature and used to catalyze the light olefin dimerizations and oligomerizations since the eighties. JP 57014538A teaches selective oligomerizations of isobutene into polyisobutene from a mixtures of 1-butene and isobutene using HPA or its salt as catalyst at 70° C. Only isobutene was oligomerized with 99.3% conversion and 99.9% selectivity for the oligomer. Al2O3-supported H3PW12O40 has been reported to be able to catalyze propylene-ethylene codimerization at 573 K to form pentenes with a selectivity of 56% (butenes 17%, hexenes 27%). Propylene oligomerization proceeded on various kinds of salts of H3PW12O40.
The activities of the salts decrease in the order Al>Co>Ni, NH4>H, Cu>Fe, Ce>K. As expected both selectivity and conversion of such supported HPA catalysts are low.
Previously, a process of oligomerizing olefins using supported HPA catalysts to produce synthetic lubricant has been reported. In addition, oligomerization of a mixed C6 and C8 olefins with HPA catalysts has been reported.
JP 2005015383 discloses an oligomerization process catalyzed by a supported HPA catalyst for selective productions of trimer of isobutene.
Teflon modified H4SiW12O40/SiO2 can be used to catalyze dimerizations of isobutene in fixed bed reactor with continued isobutene flow. However, the isobutene conversion was relatively low and selectivity towards C8 olefins was poor.
Silica supported water soluble HPA salts, such as Li/Na/K salts of HPA, had been disclosed by Kamiya for butene dimerization/oligomerization. However the activity and selectivity of such catalysts are poor with only 26% of dimer, 23% trimers and 4% tetramers when the total conversion of isobutene at 97%. Direct supported HPA or its salts are prone to leaching and deactivated quickly.
Performance of catalysts more sufficient for industrial processes is important. Therefore, there is a demand for development of a novel catalyst free from leaching and having higher activity.
As described herein before, there is a need to develop other means to replace MTBE and ethanol in gasoline as an octane enhancer. Also there is a need to replace MTBE, ethanol or alkylates as RON stabilizers or enhancers.
However, there are issues involving maintenance, leaching out of catalysts, lower inefficient activities of catalysts, and poor selectivity which need to be addressed.
Further, solution HPA can be used to catalyze the light olefin oligomerizations effectively. However, such catalysts are corrosive in nature and require high maintenances. In addition, supported HPA have been used in selective isobutene dimerization/oligomerization to produce isobutene dimers/trimers. Such catalysts are prone to be leached out, deactivation and having lower activities comparing to free HPA.
The present invention provides metal salts of heteropolyacid unsupported solid catalyst compositions useful for the production of butene dimers and/or oligomers through dimerization and oligomerization of mixed butenes.